Detergent additives have been used for decades as components of lubricating oil compositions. In recent years, however, there has been an increasing interest in the use of hydroxyaromatic carboxylate salts, especially salicylates, as essential components of so-called “low SAPs” (Sulfur/Ash/Phosphorus) automotive engine oil lubricants. For example, U.S. Pat. No. 6,569,818 discloses low sulfur, phosphorus and sulfated ash content lubricating oil compositions containing non-sulfurized alkali metal or alkaline earth metal salts of an alkylsalicylic acid.
In addition to the non-sulfurized alkali metal or alkaline earth metal salts taught in U.S. Pat. No. 6,569,818, sulfur-containing hydroxyaromatic carboxylate compositions are also known.
U.S. Pat. No. 2,311,931 discloses metal salts of alkyl or cycloalkyl salicylates sulfides having both excellent detergent and excellent anti-corrosive action when dispersed in lubricating oils and thereby having a single additive effective to inhibit corrosion, sludge and varnish formation, ring sticking and other difficulties experienced in lubricating oils serving in a heavy duty capacity.
U.S. Pat. No. 2,256,443 discloses a sulfide of an alkyl-substituted hydroxyaromatic carboxylic acid salt having increased pour depressant and viscosity index improving properties. The improved antioxidant properties are particularly significant in retarding the development of acidity in certain types of oils and under certain conditions of use.
U.S. Pat. No. 2,366,873 discloses a sulfide of an alkyl-substituted aryl metal oxide. These sulfides of alkylated aryl metal oxides are characterized by the presence of at least two aromatic nuclei, in which the oxygen of the metal oxide group is attached to the aryl nucleus, which are interconnected by at least one atom of an element selected from the group consisting of sulfur, selenium and tellurium. The compounds exhibit increased effectiveness in retarding the deleterious effects of oxidation in lubricating oil.
U.S. Pat. No. 2,366,874 discloses a metal salt of an alkylated hydroxyaromatic (phenol) sulfide. This compound is a condensation product of an alkyl-substituted aryl metal oxide in which the oxygen of the metal oxide group is directly attached to the aryl nucleus and in which at least two alkyl-substituted aryl nuclei are interconnected by at least one atom of sulfur.
U.S. Pat. No. 3,410,798 discloses basic metal salts of phenol or salicylic acid sulfides prepared by reacting a phenol or salicylic acid, or a salt thereof, with sulfur and an alkaline earth base at a temperature of about 150° to 200° C., in the presence of a carboxylic acid salt thereof and a polyalkylene glycol or alkylene or polyalkylene glycol alkyl ether. The products are useful as detergent additives for lubricants.
U.S. Pat. No. 3,595,791 discloses basic metal salts of salicylic acid sulfides prepared by reacting salicylic acid, or a salt thereof, with sulfur and an alkaline earth base at a temperature of about 150° to 250° C., in the presence of an alkylene or polyalkylene glycol or a monoether thereof. The products are useful as detergent additives for lubricants.
U.S. Pat. No. 6,235,688 discloses sulfurized phenates, sulfurized salicylates, salts of sulfurized multi-hydroxyl aromatic compounds and chemical and physical mixtures thereof.
European Patent Publication Number 0168111 discloses sulfurized metal aliphatic hydrocarbon-substituted salicylates, characterized in that an aliphatic hydrocarbon-substituted phenol is sulfurized and the resulting product is transformed into an alkali metal salicylate with an alkali metal hydroxide and carbon dioxide.
European Patent Publication Number 0168110 discloses sulfurized overbased, metal aliphatic hydrocarbon-substituted salicylates by sulfurization of an aliphatic hydrocarbon-substituted salicylic acid or a metal salt thereof with a sulfur halide, and subsequently by transforming the reaction product into an overbased metal salicylate.
European Patent Publication Number 0168880 discloses sulfurized overbased, metal aliphatic hydrocarbon-substituted salicylates, characterized in that an aliphatic hydrocarbon-substituted salicylic acid is transformed into overbased metal salicylate having a basicity index of at least 1.5 by means of a basic metal compound and with carbon dioxide, and subsequently the overbased metal salicylate is sulfurized by heating with elemental sulfur.
The above references, however, teach hydroxyaromatic carboxylate compositions containing relatively high levels of sulfur, which are not desirable in formulating low SAPS oils. Sulfur contained in the fuel or lubricating oil is converted to sulfuric acid and sulfates which are often corrosive. Hence, the need for low levels of sulfur. However, it is often difficult to achieve low sulfur content levels without compromising the effectiveness of detergent additives in the fuel or lubricating oil. Effective low sulfur detergents are therefore highly desirable.